Such a device can be found in the prior art. For example, as mentioned in German laid open document DE-AS 15 74 292, two detector electrodes are provided in the area of the edges of the strip which are capacitively coupled onto the strip and create such a capacitance field. A lateral movement of the strip causes changes in that field which are then used to control the lateral positioning of the strip. C-shaped detectors have been used in industry and are known to provide a compact structure.
This device of the German laid open document is not very sensitive to changes of the strip position. Its insensitivity is caused by the fact that the detector electrodes not only create capacitances with the strip edges (useful capacitance), but also with every nearby machine part (extraneous capacitance). Such extraneous capacitance is connected in parallel to the useful capacitance and thus reduces the relative sensitivity of the sensor to strip position changes. Since the interfering machine parts near the detector electrodes are, in general, not symmetrically arranged, other capacitors have to be connected in parallel as compensation. This, however, further reduces the sensitivity of the system.
The low sensitivity to strip position changes is also mentioned in German laid open document DE-AS 15 74 292. To remedy this, the capacitance of the coupling fields is supplemented to an oscillator circuit by means of an inductive reactance. With this, the strip position changes influence the resonance frequency of the oscillator circuit. The quality (Q) of the oscillator circuit can influence the sensitivity of the total arrangement to strip position deviations. High quality gives high position sensitivity.
In this way, the sensitivity of the device can be increased; however, the disadvantage of this, as well as other known techniques for capacitive strip position detection, is that the detector electrodes not only react to the deviations of the controlled strip, but also to all other nearby conductive parts. The influence of machine parts can be compensated insofar as the strip can be center guided. However, compensating capacitors connected in parallel to the detector capacitances prove to be disadvantageous to the position sensitivity of the system since the capacitance change caused by a strip movement represents only a small fraction of the total capacitance of the total capacitance of the system.
If the machine has moving conductive parts, compensation by means of a fixed-shunt capacitor is not possible. Also, the human body represents a sufficient conductive counterelectrode so that each person moving around or near such sensor devices causes interference with the system. This situation is very disturbing to the maintenance and operation of such systems since interference with strip control can easily result in damages to the material in-process and to the system. For these reasons, the techniques described in the aforementioned German laid open document are not widely used in the industry. Instead, optical scanning devices for the strip edges are preferred, even though their susceptibility to interference from dust and dirt is high.
It is possible to shield the sensor against moving conductive parts and/or people. But, shielding of the detector electrodes against the described interference creates even more parallel capacitances and results, again, in sensitivity reduction. For that reason, the distance between the detector electrode and its shield should not be less than the distance between the detector electrodes and the strip in order to maintain enough system sensitivity. That causes the size of the shield to grow and in many instances, a lack of space makes the installation of such a shield impossible.
An object of this invention is the creation of a capacitance sensor which detects the position of a moving strip and is at the same time substantially insensitive to external interferences and very sensitive to strip position deviations. It is also an object of the invention to be of compact construction.
Starting with a brief description of the technique, the method and apparatus of this invention is as follows: The detector electrode(s) in the area are turned away from the extraneous coupling fields and are surrounded by at least one shielding electrode. This shielding electrode(s) is connected by way of an amplifier(s) with a gain of unity to the detector electrode(s) in such a manner that the potential of the shielding electrode(s) is always the same as the potential of the detector electrode(s).
The described technique assures good shielding of the sensor with shielding electrodes close to the sensor. This not only saves space, but eliminates nearby interfering influences while the sensitivity is maintained. The sensitivity, e.g., the change in useful capacitance in reference to the strip position, can in this way be increased. Forming an oscillator circuit as suggested in German laid open document DE-AS 15 74 292 is no longer necessary and the large coils are no longer needed, which is in the interest of compact construction.
With the C-shaped sensor electrodes, it is possible that a deviation of the strip from the required position may not be detected. This can occur when one edge of the guided strip moves slightly away from its sensor electrode, but at the same time, comes closer to one leg of its electrode while the other strip edge moves toward its electrode without coming closer to one of its legs. In a case such as this, it is possible that the coupled capacitances between each of the strip edges and their proper sensor electrodes stay the same, in spite of a shifted strip position and, for that reason, the deviation is not detected.
Disclosed are embodiments of the invention which do not require the use of C-shaped sensor electrodes, but which rather use flat or planar electrodes, which embodiments eliminate the previously described difficulty. In this form, the strip edges themselves do not represent capacitance-connecting counterelements to the detector electrodes, but only influence the capacitance coupling between the detector electrodes and the counterelements, which, on their part, stay relatively fixed with respect to each other.